text:
How are "Owner-Produced Parts" handled under FAA regulations?
FAR 21.9 provides the ability to install parts that were "Produced by an owner or operator for maintaining or altering that owner or operator's product". This seems like a pretty broad scope of authority for aircraft owners.
Under what circumstances are owner-produced parts typically used, and what requirements are imposed on the part (and the owner producing it)?

What kind of part? A 3D-printed plastic seat belt holder for your Cessna will receive one level of inspection, a hand-forged fan blade for a Trent 970 will receive a bit more scrutiny.

and a piece of duct tape to cover the hole in the wing of your Cessna, hmmm.  (and yes, I've seen that, had I known more about aircraft maintenance requirements at the time I'd not have climbed on board that thing...).

Wasn't duct tape. There is an FAA-approved product called speed tape that looks almost identical ( 2 inches wide, silver ) and is commonly used for temporary repairs. If you've never seen the proper stuff up close you won't know the difference.

@paul if there's a difference describe it in your answer. (As far as I'm aware there isn't: you have to demonstrate equivalence in either case...)

Duct tape from Home Depot and aerospace-grade aluminized adhesive tape from 3M are not equivalent in any way other than both are tape, and 2 inches wide.

@paul I was responding to your first comment where you assert there are different levels of inspection - is this by regulation, in an AC, etc.?  The use of speed tape is not at all relevant to this question (as far as I'm aware neither speed tape nor duct tape qualifies as an owner-produced part under the FARs), but it might be an interesting subject for a separate question & answer.

Ah - misunderstood the target (your comment appeared right under the duct tape). My experience with the relevant authorities shows they really don't care about largely cosmetic but still technically "requiring certification" parts on light aircraft. We made our own interior panels for a Cessna 206 and the comment was "nice job". The step bolted over the wheel required a field-approved STC.

A while back I needed to replace a flap track on my old Cessna 340.  The company that produced them was still in business, but did not have the part.  I had them produced the part according to their specs.  I received a traveler (the little white card) for it, that said the item was produced at the owner's direction referencing the FAR above and their STC.  My A&P replaced the existing track with the new part and made an entry in the log book stating that they replaced the old track with a new track manufactured per the FAR.  I had a FAA review of the logs at a later date with no issues.

@paul not to be confused with the silvery metallic tape you can get at the hardware which I cannot find the name of but presumably isn't aviation certified?

@Michael If you're talking about the stuff you're supposed to use on duct work (which is actually metal tape) we call that "HVAC Tape", "Foil Tape", or "Alumatape". (Like "duct tape" this generally isn't something you'd use for exterior repairs, but 3M makes at least one line of it (3M 433 tape) that's suitable for use on cabin duct work.)

@voretaq7 Yes, that's it!  Wikipedia lists speed tape but not any of those names for HVAC tape.  And why people use Duct Tape for ducts (or why it's even called that) I have no idea - it's absolutely rotten for ducts IMO.

The July/August 2002 edition of FAAviation News (now FAA Safety Briefing) has a detailed article on owner-produced parts, including advice on best practices if you want to make your own parts. It does confirm that owners have very broad authority to do so, provided that they document everything properly.
Typical reasons for producing your own parts are (I've paraphrased them from the article here):

The aircraft has been out of production for years
The aircraft is an orphan without a Type Certificate
There is no technical support because the aircraft is so old
There may be long delays in getting parts from the manufacturer
The price of official parts is extremely high

As for requirements of the part itself, the article list four characteristics that a part must have:

The part must be properly designed
The part must be produced to conform to the design
The part's production must be properly documented
The part must be properly maintained

The owner must be involved in the production but doesn't actually have to do the manufacturing himself:

The aircraft owner must participate in the manufacture of the part
in at least one of five ways for it to be considered an Owner Produced
Part.

The owner provides the manufacturer of the part with the design or performance data.
The owner provides the manufacturer of the part with the materials.
The owner provides the manufacturer with fabrication processes or assembly methods.
The owner provides the manufacturer of the part with quality control procedures.
The owner personally supervises the manufacture of the new part.

More recently, in the September 2014 edition of the Cessna Pilots Association magazine, Mike Busch has an article on replacement parts that calls the current regulations on owner produced parts "extremely liberal":

“Owner-produced parts” do not require a  PMA. These are parts produced
by an aircraft  owner for use on his own aircraft, and not  offered
for sale. There is an FAA letter of  interpretation that sets forth
precisely what  an owner's involvement must be to qualify a  part as
“owner-produced,” and it is extremely  liberal. The owner need not
actually  manufacture the part himself, so long as  he provides
specifications or materials  or supervision or some other meaningful
participation in the part's production.

Unfortunately, he didn't mention exactly which letter of interpretation covers these parts but the FAA magazine article refers to an "FAA Memorandum dated
August 5, 1993" and I guess it's probably the same one. I wasn't able to find it on the FAA site.

Mike Busch's EAA article, https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2011-08_owner-produced-parts.pdf, suggests that the memo is from April 5th 1993. Could the date discrepancy explain why the original memo eluded your search?

Found the elusive memo, available for download here: http://www.velocolutions.com/FAA-owner-manufactured-part.pdf


text:
Where can I get information on ATC facility activity, like the number of flights handled in a month?
As a numbers/statistics geek I would like to be able to look up information on how "busy" particular airports or regions are.
I'm sure the FAA collects this data, but is it accessible to the general public?

The FAA's Air Traffic Activity Data System (ATADS) contains broad statistical information on activity in the national airspace system which can be used to get an idea of how "busy" a particular Center, TRACON, or Tower facility is.
The caveat is this data only encompasses aircraft talking to Air Traffic Control, so it won't be able to tell you anything about uncontrolled fields except possibly what you can glean from Center/TRACON activity information for the greater area.

I'm not sure where they get their data from, but Fltplan.com has annual reported data for each airport in their Airports & FBO's database:
For example, this is for KBRY (which is uncontrolled):

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text:
How do airports determine when they should change their runway numbers?
Runway numbers in most of the world are based on the runway's orientation relative to magnetic north.  For example a runway with a magnetic heading of 135° to 144° will be numbered 14, and one with a magnetic heading of 145° to 154° will be numbered 15.
Runway numbers are occasionally changed due to changes in the Earth's magnetic field.
At my home field Runway 14/32 is oriented 145.7°/325.7°, per the FAA's airport diagram, and it has been this way for some time (I first recall noticing it over a year ago).
My understanding is that this runway should, at some point, be renumbered 15/33.
How does the FAA determine when to request that an airport change its runway numbers?
Is the decision to update the runway designation made by the FAA, or by the airport's management?

FYI, this airport in Missouri has changed its runway numbers to 12/30. Southwest practice short-field landings here.

@Farhan: Did they start practicing there after one of their flights landed there by accident, or was that what you were actually referring to?

@Sean That was joke. There was a single incident with Southwest airliner.

Implementing changes to runway numbers is quite complicated and requires a lot of different parties to get involved.  There is a ton of related information in FAA Order 8260.19E and it has the details.
Simple Version
According to an article by the NBAA called "How Changes in Magnetic North Are Impacting Airports":

“Adjustments to runways like this and to navigational aids are
ongoing,” according the Kathleen Bergen, a spokeswoman for the FAA
Southern Region. “Every five years, the FAA reevaluates shifts in the
pole—its magnetic variation—and makes changes to runways and flight
procedures as needed,” she said.

Medium Version
NOAA, NOS, and NGS (see below) provide the magnetic variation information, and publish it every five years.  They work with a bunch of different offices, and different branches of the military as they make adjustments.  When a determination is made that a runway number should be changed, they coordinate with several other groups (such as AeroNav Products, OSG-FPT, the applicable Airportt Traffic Service Area Office, and the Airports Division), to choose the actual number used, based on "careful consideration and evaluation of a number of factors" and then to make the actual changes in charting, runway markings and signs, etc.
They are supposed to change it when it is more than three degrees off from what it is should to be.  AC 150/5340-1L - Standards for Airport Markings contains the official runway designator rules (paraphrased):  If the actual magnetic course of the runway ends in 5, it can use either number, otherwise you round to the closest runway number (there are some variations for special cases like parallel runways and such, but this covers most runways). So in your example, 145 is still valid as runway 14, and they wouldn't need to change it until it exceeds 148 deg.  In your case, the annual rate of change for Farmingdale airport (as of 2010) is 0.0 deg East, so I don't see that happening for awhile.
Long Version - Quotes from FAA Order 8260.19E (above)

Section 5. Implementing Epoch Year Magnetic Variation (MV)
a. Background. The National Oceanic and Atmospheric Administration
(NOAA), National Ocean Service (NOS), and the National Geodetic
Survey (NGS), for all areas of the United States and its territories
for application to navigation charts and maps, is the source for
magnetic variation (MV) information and tools for establishing
magnetic variation. Changing values for MV are tabulated and published on
a 5-year epoch basis; e.g., 00, 05, 10, 15, 20, etc. In order to
assist in stabilizing the National Airspace System (NAS), a fixed
value of MV is assigned to each  navigational aid and airport as the
Magnetic Variation of Record. This value is applied to true
directions to obtain the magnetic values for radials, courses,
bearings, and headings published in instrument flight procedures.
Periodic updating of the MV assigned to navigation facilities is
required to maintain reasonable proximity of alignment with the
earth’s ever-changing magnetic field. ...
b.
Participating Offices. Management and control of Epoch Year MV values
require action  by the following offices:
(1) AeroNav Products.
(2) Military Organizations.
(3) National Flight Data Center (NFDC).
(4) Western, Central, and Eastern Technical Operations.
(5) Western, Central, and Eastern OSG-FPTs.
(6) Regional Airports Divisions.
...
f. Regional Airports Division/Airports District Office (ADO).
Coordinate with the  applicable OSG-FPT prior to establishing or
revising runway designator numbers for an airport  having one or more
instrument approach or departure procedures, to determine the
appropriate MV to be applied to the runway true bearing. Determination
of the runway designator number should be a matter of joint agreement
with AeroNav Products, and be accomplished sufficiently in advance to
allow for procedural amendments. Take appropriate NOTAM action if
repainting of an affected runway has not been accomplished on the
required date.
...
2-18. Guidelines. The identification and selection of navigational
aids or airports as candidates for revision of MV of Record require
careful consideration and evaluation of a number of factors - as the
impact of MV changes can be considerable. The applicable Air Traffic
Service Area Office may have to initiate or revise published air
traffic procedures; the Technical Operations Service (AJW-0) is
directly involved in facility rotations and requires proper
coordination. The Airports Division, or appropriate military
authority, may have to arrange for repainting of runway designator
numbers [see paragraph 8-58e(2)(e)].
a. MV versus Epoch Year Value. When the difference between the MV of
Record and the nearest future Epoch Year value of any navigational aid
or airport is 3 degrees or more, the MV of Record must be changed to
the nearest future Epoch Year value.

There is no real need to change the runway identifier just because the variation changed. Sometimes this is done but you'll find enough places aroud the world where there is up to a 10 degree offset (maybe even more) between the numbers on the runway and it's magnetic orientation. At some biger airports with more then three parallel runways this is even done by purpose because if you already have xx left, center and right you can't call the fourth "right right" but what you do is you name them xx left and right and xx+1 left and right see for exampel LAX.
Now lets think about the use of the runway numbers. I only need them to x-check if I'm on the right runway prior to departure, to have a reference while approaching an airfield and to quickliy calculate the x-wind - my math isn't so good that five degrees would make a difference ;) and the other things are for sure important but also work out with some offset (I have never seen intersecting runways with only 10 degrees difference in orientation where a confusion would be possible)
There is no source where I found this but in my eyes it's very dependent on the importance of the airfield if they change it when there are more then 6 degrees offset or not.

Another interesting example regarding parallel runways is ATL. It actually has 5 parallel runways: 8R/26L, 8L/26R, 9R/27L, 9L/27R, and 10/28, all with a 095/275 magnetic heading.

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text:
How significant is antenna drag on light GA aircraft?
Antennas sticking out of an aircraft obviously increase profile drag, but the folks who design antennas have done a lot to improve aerodynamics: High performance aircraft can have antennas mounted inside of fiberglass components like wingtips, and flush-mounte antennas are available for transponders and DME equipment.
For slower light GA aircraft more aerodynamic antennas are also available (e.g. blade-style transponder antennas are available which produce about 80% less drag than older spike-and-ball style antennas), measured at 250 MPH.
Since the average light GA aircraft has a VNE below 250 MPH and isn't generally covered in antennas, how significant is the antenna drag on a typical light GA airframe?  For example, would stripping the ~6 pounds of antenna drag from your average Cessna make an appreciable difference in airspeed?

if you know you have 6 pounds of antenna drag and you know the pounds of thrust of the engine then you can get a rough estimate of how much faster you go faster

From an aerospace engineering perspective:
The drag coefficient ($C_D$) of a round antenna is can be estimated by around $.5$ depending on shape, while for an streamlined shape it's around $0.045$.

(Source)
Drag is defined as:
$$D=\frac12C_D{\rho}V^2A$$
Thus, say you have a 0.01 m wide antenna 0.25 m long, for an aircraft moving at 50 m/s. At sea level conditions, air density $\rho=1.225 \mathrm {kg/m^3}$. Say that $C_D=0.5$. If my calculations add up, it's around $1.9 \mathrm{N}$.
Say you have the same antenna, for the same aircraft aircraft moving at 50 m/s. Air density is $1.225 \mathrm{kg/m^3}$, but this time the antenna is streamlined with $C_D=0.04$. This results in a force of only $0.15 \mathrm {N}$.
(Updates: Originally the formula included $\frac12$ twice which was wrong, and that the force was given in kg, rather than newtons; my apologies.)

Does anybody know a link to instructions to find out how to format formulas correctly?

At the moment MathJax is not enabled on this site because of the processing overhead and the related long page loading times. See http://meta.aviation.stackexchange.com/q/145/19 for the Meta discussion

You can do a lot to improve readability of formulae using <sub>, ρ etc. I took the liberty of adding those.

I also took the liberty of omitting "/2" from the drag formula, as it was incorrect, but the factor appears to have been edited back in.

@marcksThomas you are correct. i don't know who edited it back in. (so used to seeing it with ½ upfront.

Manfred, what's happening in Delft? Why don't you just use Newtons as a result from your calculations. Now it is confusing because your units don't match up. To express force as mass isn't the way to do AE  :-). Also thrust is not defined a power available * velocity. Delivered power is defined as thrust * velocity (TAS). Thus thrust = delivered power / velocity. Note that the rated available power from your engine is more than the delivered power because you loose some in transmission and propeller efficiency.

And the same for drag: drag = Pr / velocity

In fact if my calculations are correct, the result of your first calculation should be 1.9 N instead of kg. You over estimate the drag by factor 10 almost. Correct it before Voretaq7 cuts of all antennas of his aircraft! ;-)

I like the formula and estimates of drag coefficient and force, but I think a layman's example would add a lot to this answer. Pick an airspeed and say x-antenna-scenario would result in a reduction of y-knots, or would require z-extra horsepower to maintain the same speed.

@Bret Did a little of that earlier but I'll try to reinsert that later when i can get my formulas right :D

@DeltaLima MathJaX is now enabled (see the edited answer!)

Given how abysmal your average GA plane is in the aerodynamics department, the amount of drag added by antennas is almost nothing.
I have some numbers for a Cherokee 160 from an aircraft performance class -although I make no claims that they're 100% accurate- and I'm seeing between 250-360 lbs of drag during level flight depending on the airspeed. Add your 6lbs of antenna drag onto that, and in order to maintain the same airspeed you need at maximum (a density altitude of 10,000 feet) 3 additional horsepower. At sea level it barely registers a change at all.

I think the drag numbers for the Cherokee series are pretty accurate (as is your math on the horsepower at typical GA cruise speeds). Working with about 130KIAS my math came out to between 1 and 4 percent of engine horsepower going to fighting drag (much of the variation from stuff like propeller pitch & cruise power setting) the antenna drag, or roughly 1-3 knots if you completely removed and smoothed the antennas. Obviously that's the mathematical ideal - in the real world I'd expect less substantial gains.

It's worth noting though that as lousy as light GA aircraft are in the aerodynamics department some antennas are worse - like these "Flying V" beasts. I don't even want to think about the profile drag on that :-)

@voretaq7 That thing looks like it might actually generate lift though!

@voretaq7: according to this document, removing the antennas gained 4 knots in cruise speed (probably KTAS), so seems to be accurate.


text:
FAA Commercial Pilot Knowledge Test: Sign Question
This image appears in the FAA Test Supplement Commercial Test FAA-CT-8080-1E but I am unable to find it in the AIM or the Pilot’s Handbook of Aeronautical Knowledge. I looks like a don’t taxi here pavement marking, but I’d like confirmation of my guess.
The closest I can get to an official explanation is in the AIM 2-3-2 b

Markings for taxiways, areas not intended for use by aircraft (closed
  and hazardous areas), and holding positions (even if they are on a
  runway) are yellow.

But the examples don’t match this graphic.

This sign is a taxiway ending marker.  I retrieved this graphic from the CFI Notebook Website.

Knowing what it is called helped me find it, It is in Appendix C of the Pilot's Handbook.

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text:
Why does TCAS not issue turning commands as Resolution Advisory?
The TCAS (Traffic Collision Avoidance System) is capable of issuing RA (Resolution Advisory) when a mid-air collision is imminent by telling the pilots to "climb", "descend", "increase climb" etc.
Why does the system only issues commands in the vertical direction, but not the horizontal direction? It seems an oversight in the design of the system to not make use of all available maneuver options to resolve a traffic conflict. Why can't the system tell the pilots to "turn left", "increase right turn" etc.?

Side note: future versions probably will

TCAS gets fairly accurate altitude information from the same source as the transponder. Thus any extrapolation from this information can yield a fairly accurate prediction of whether two aircraft may collide, and provide more certainty that changes in altitude will avert any conflict.
Range information is obtained by measuring the round trip time of the interrogation and response. This will also be fairly accurate.
Direction information is obtained by using the TCAS antenna to detect the direction of responses from other aircraft.
These values are then extrapolated to provide an expected path of other aircraft. Most airliners are required to use TCAS II which only provides the vertical guidance. TCAS III was envisioned to provide horizontal guidance as well, but:

it was judged by the industry to be unfeasible due to limitations in
  the accuracy of the TCAS directional antennas.

Even if TCAS can be accurate to 3 degrees in direction, this translates to over 300 feet of uncertainty at 1 nmi.
TCAS IV was planned to use more accurate GPS information, but with ADS-B becoming more widely adopted, this development was stopped in favor of some future system that can use existing ADS-B technology.
Another point is that airplanes are generally larger in the horizontal plane than the vertical, and thus would need to change course more horizontally to avoid collision than they would need to vertically. Especially as planes get larger, they also tend to be more maneuverable in pitch than in turns.

The vertical dimension is already sufficient to resolve a collision. It also makes resolution advisories (RA) much simpler to comply with. There's no need to make the software, RA and following the RA more complex than it needs to be.

Can you provide some statistics / research / authoritative source, about "vertical dimension is already sufficient to resolve a collision"?

Well the track record of TCAS shows that it works with just vertical resolution, and I'm sure they expected it to when they designed it. But this begs the question, if you will only use one, why choose vertical over horizontal?


text:
Should spoilers and reverse thrust be deployed before nose gear touches down?
On airliners, should spoilers and reverse thrust be deployed before nose gear touches down?
I used to thought that deploying them will affect the handling characteristics, so it is best to wait until all wheels are on the ground; but I've seen pilots do it both ways. Does it depend on landing distance and runway condition?

The main purpose of a spoiler is to "dump" the lift. e.g. a spoiler disrupts the airflow around the wing such that the wings does not produce lift in the spoilered area. Ideally you don't want do deploy the spoilers if you would like to roll the aircraft (depends on location of the spoilers). So you deploy them as soon as both rear wheels are on the ground.

It depends on the aircraft (and conditions). In some aircraft like the MD-11, the deployment of ground spoilers causes a nose up pitching moment- in such a case, the spoilers should be deployed after nose landing gear touches down.
An example is the incident in Hawaii where the aircraft, a MD-11 exhibited a pitch up tendency during landing and the pilot had to initiate a go around. The factual report noted:

The MD-11 Flight Crew Operating Manual, Volume II, Landing Roll Procedure, contains the following note:
Ground spoiler deployment causes nose up pitching moment. This effect is most noticeable at aft centers of gravity. It is important to check the nose up pitching tendency with forward pressure on the control column and smoothly lower the nose wheel to the runway.

In some cases the deployment of thrust reversers could also have similar effect. In such cases, the thrust reversers and spoilers should be deployed after the nose wheel touches the ground, while in other cases, it is done after the main landing gear touches down and before the nose gear touches down. From FAA Airplane Flying Handbook:

... some airplanes tend to pitch noseup when reverse is selected on landing and this effect, particularly when combined with the noseup pitch effect from the spoilers, can cause the airplane to leave the ground again momentarily. On these types, the airplane must be firmly on the ground with the nosewheel down, before reverse is selected.
Other types of airplanes have no change in pitch, and reverse idle may be selected after the main gear is down and before the nosewheel is down.

There are other issues as well- deployment of thrust reversers before the nose wheel touches the ground means the pilot may not have sufficient authority for steering the aircraft in case one of them doesn't deploy. To prevent this some aircraft restrict the use of full reverse thrust till the nose landing gear is on ground (though idle reverse is permitted once the main gear touches down).
Also, for aircraft having engines in rear, the deployment of clamshell type thrust reversers can have clearance and debris ingestion issues which can limit their deployment. Also, the effect of spoiler deployment on the aircraft aerodynamics has also to be considered. For example, Boeing states:

The MD-80 and MD-90 in-flight spoiler lockout mechanism prevents the undesirable flight control configuration of deployed spoilers in either the speed brake or ground spoiler mode when flaps are extended.

There are also other aircraft (like Tu-154) where the thrust reversers were deployed before touchdown. So, all in all, it is better to consult the particular aircraft manual to decide when the spoilers/thrust reversers are to be engaged.

The MD-11 has auto spoiler deployment on wheel spin up, so the warning about pitch up is an advisory only and not meant to delay deployment after touchdown.

See https://en.wikipedia.org/wiki/Air_Canada_Flight_621 for what happens when there is confusion about arming/deploying spoilers

Does the DC-9-80/-90 ("MD-80/-90") have special operating limitations when landing with an inoperative in-flight spoiler lockout mechanism?

Spoilers
Spoilers are designed to be automatically deployed upon main wheel touchdown. There is no pilot selection unless they forget to arm the system or it is unserviceable.
Delaying spoiler deployment risks bouncing and reduced braking, as spoilers put weight on the wheels.
Some aircraft like the MD-11 with a relatively high-aft center of mass, have a two-stage auto-spoiler deployment, so variations will depend on the airliner. No pilot input is needed if the system is operative.

Reverse Thrust
Reverse selection is manually controlled and there will be some variation. Many pilots like to select reverse prior to nose wheel touchdown because reverse is more effective at higher speeds.
The MD-11 also delays the full deployment of the number 2 engine reverse, again no pilot input is needed, the pilot just selects the reverse on all three engines, and the system automatically takes care of the rest.

I didn't realize I posted it that way. I answered using my phone and got mixed up.


text:
Is there a checklist for lightning strike?
Is there a specific checklist or procedure to follow, after a confirmed or suspected lightning strike on the aircraft?
Or is a basic scan around the cockpit (plane is flying, no warning lights, no circuit breakers popped) sufficient?

From a flight crew or maintenance perspective?

For maintenance: Boeing Aero magazine ⌁ and ⌁ Lufthansa Technik.

Which aircraft type/category are you asking about?

For the aircrew, at least in the aircraft that I've flown, there is no defined checklist.  You scan and then deal with anything that occurs (inoperative equipment, pressurization issues, whatever you get) with whatever checklists apply: if the lightning knocked out a hydraulic pump, run the checklist for the inop pump, etc.
Once you're on the ground, maintenance has a HUGE checklist to work through, essentially checking every surface of the aircraft for damage and verifying that every piece of equipment still works.  The more that the crew can tell them (i.e. radios & instruments unaffected, no equipment stopped working, pressurization normal, etc), the more of it they can sign off from the crew observation.  But even then, it's still pretty lengthy.

Agree. The "checklist" is something like: (1) What was that? (2) Was...was that lightning? (3) Does everything still seem to work? (4) Did anything catch on fire or fall off [look outside]? (5) If F/O appears nervous, look at F/O confidently and say "it happens." If no F/O, jump to step 6 immediately. (6) Sip coffee.


text:
How can I get prior days weather information?
If I want to get airmet information for prior days, like say last week or last month, how can I do that? Specific instructions would be appreciated.
(So, this would be information like winds aloft and ceilings.)

Looks like this may be a "resource location" question (and thus off topic).

Weather Underground used to have a good archive, at least for METAR reports, which would give you ceilings. Dunno if they still do, nor about winds aloft. Maybe ADDS or NOAA - they have a lot on their servers.

Related: https://aviation.stackexchange.com/questions/22086/where-can-i-get-a-history-of-metars .  Note the comments under q and a's.  Also https://aviation.stackexchange.com/questions/14596/is-there-a-database-of-airport-ceilometer-raw-data.

You can get past weather for particular weather reporting stations from the National Weather Service. The help page has several different options, including how to obtain certified weather (used in accident investigations and litigation). The website is located here.

Try this link and select Upper Air. Also available are Surface Weather (metars), Satellite imagery, and Radar.  The Upper Air selection allows user selection and review of individual maps of winds aloft for any image over the last 10 days, or as an animation of frames for these maps over a time period of up to the last 10 days. The site is the Research Application Laboratory, University Corporation for Atmospheric Research, NCAR.
The Naval Research Laboratory, Monterrey, may provide other imagery of interest regarding upper air maps and animations at this link.


text:
part NCC aircrafts in europe and pilots flying with FAA licenses
Good morning,
i know that since 2020,2021,   FCL licenses are necessary for an european pilot to fly an aircraft registered in a third country BUT based in France with part NCC; it seems that we can have a validation of an FAA license for one year, and another year if we follow a formation to get the EASA CPL IR exams...
my 2 questions are :
Is the request for the validation is difficult to obtain with the French DGAC ?
And what about a non european resident flying this aircraf with only FAA licenses ?
Thank you for your cooperation
Regards


text:
Telluride (KTEX): Why is the localizer antenna located where the glideslope should be?
Telluride (KTEX) in Colorado, USA:

Source
As visible, the localizer antenna for RWY 09 is located where the glideslope antenna is usually found, abeam the touch down zone. The localizer is normally seen at the end of the runway, aligned with the runway center line.

Why was the localizer moved close to the TDZ at Telluride airport?
What is the practical impact for the pilot to have the localizer beam distant from landing path?

@RalphJ -- it's neither -- it's a straight-up LOC.  I wonder what'd happen if you added a GS to this config, though...

The localizer for runway 09 is offset by 3°, almost certainly because of terrain. That would be a typical reason for a non-standard installation.
The approach plate for the LOC RWY 09 approach notes the offset (more than 3° and it would be an LDA approach):

The final approach course for the LOC 09 is 093°, whereas for the two RNAV (GPS) approaches it's 096°, which is also the current magnetic heading of the runway. The Chart Supplement has various notes indicating that terrain is an issue (and RWY 09 has a displaced threshold) so everything indicates that the localizer is offset slightly for that reason, presumably to avoid signal interference.
As for practical issues for pilots, correcting for a 3° offset shouldn't be a problem. A bigger consideration seems to be that the MDA on the LOC 09 approach is more than 2000' AGL so if the pilot breaks out late he'll very likely have to circle to land.

I've flown in here quite a few times, and it is absolutely offset due to terrain to the North, which is quite impressive!  There is also a cliff just prior to the runway threshold while on approach to runway 9, so don't land short.


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Why do fractional programs seem to have so few hours?
Fractional ownership programs I have seen often seem to have a relatively small hour allotment. For example, a typical arrangement might be a 16 way ownership with 50 hours each. So that is a total of 800 hours per year for all the owners.
Of course, a calendar year has in it 365 * 24 = 8760 hours in it. Which means that less than 10% of the available time is allotted to the owners.
Even if we allow 1000 hours for positioning and 150 hours for maintenance and 250 for cleaning and prep, that is still only 2200 hours total, about 25% of the available hours. Let's further subtract 10 hours per day, or 3600 per year for "late night" hours that people would not want to fly. We still have 3000 hours unaccounted for.
What is going on here? Do fractional operators cheat the owners by chartering out those 3000 extra hours? What is happening to those hours?

50 hours is actually a lot of flying for one person in a year, especially when you factor in the cost of those hours. The fractional aircraft I was a part of, with 8 people was lucky to get 50 hours a year total between all the members. Bad weather days, winter, time out for annual/maintenance (annual typically takes a week or two out of the schedule), oil changes (about 1-2 days). Factor in that if you fly 800 hours a year, you'll be doing engine overhauls every ~2.5-3 years (if following TBO)... 800 flying hours for a private aircraft is a lot of wear.

@RonBeyer   Also, the cost of all that maintenance is a big incentive for shareoplane groups to keep the hours down.

Since it's a leisure-time activity, most of the co-owners will want to fly on weekends, because they are at work on workdays. So you can't allocate as many hours, because you'd have problem scheduling them.

Not to mention that being a 'fractional owner" doesn't mean you are without an hourly cost. Costs for fuel, insurance, overhaul, hangar, and maintenance can be from $100/hour to over $300/hour depending on what type of aircraft you have. This means the 50 flying hours still may cost you up to $15k in disposable income a year if you want to fly something like an SR22-GT, even higher for things like a Vision jet.


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PC-24 has a cargo capacity of only 400 pounds?
I went to book a PC-24 with 3 passengers weighing a total of 450 pounds and 700 pounds of luggage for a total of 1,150 pounds. I thought this was doable because the payload capacity of a PC-24 is stated as 2,500 pounds. However, the operator came back and said I could only have 400 pounds of luggage. Nowhere could I find this limit in the specifications. Where is this limit defined?

Max payload is 2,500lbs but Min payload is 715lbs with full fuel. Please include the flight routing so we can have an idea of how much fuel is required. Also, will the flight have 1 or 2 pilots?

@MikeSowsun Aircraft payload does not include fuel. "Useful load" is the characteristic that includes fuel. The useful load of a PC-24 is 6880 pounds.

Yes, “payload” does not include fuel, but fuel required for the mission determines max payload. What is the flight routing you are planning?

I get you're sore that your project won't work, but you would benefit from finding the reasons why instead of just throwing tantrums and shooting messengers.  You demand that all airplane pax+cargo capacity assume full fuel.   That would unnecessarily limit the aircraft on shorter flights where full fuel is not required. What you don't understand is the rocket equation. It's an airplane not a car, fuel is a significant fraction of total weight.

Payload is traded off against fuel, so the combined number is the limit.
The PC24 has a useful load of 6679lb, consisting of 5964lb max usable fuel plus 715lb payload with full fuel. If the operator is allowing you 850lb, he is figuring 200lb for a single pilot (400lb for two pilots) and reserving 5629lb for fuel. This gives a 2000nm range at long range speed with reserves.
You don't get 2500lb payload AND full fuel. If you want to carry the maximum payload of 2500lb you have to cut fuel to 4179lb to stay below gross. This amount of fuel cuts range to about 1500nm. Is that how far you want to go?

????? What ???? The stated payload of a PC-24 is 2,500 pounds, as it says in the question. The PC-24 is an EIGHT seat aircraft. That's in addition to the pilots.

@TylerDurden So look at the reference. The useful load is 6679 which counts both payload and fuel. You don't get 2500lb payload AND full fuel. If you want to carry the maximum payload of 2500lb you have to cut fuel to 4179lb to stay below gross. This amount of fuel cuts range to about 1500nm. Is that how far you want to go?

Payload includes cargo and people. It does not include fuel.

@TylerDurden. It would seem the sales pitch needs technical review.

@TylerDurden So look at the reference, do some math and get back to me. Max payload with full fuel is 715lb, not 2500lb. Again, if you want 2500lb payload you have to cut fuel. There is more than one constraint. This isn't that hard.

Seems all the answers are missing the point.  The question was “why can’t you take more than 400 pounds of baggage and where is that listed?”  The max load in the cargo area is 400 pounds and that is in Section 2, “Limitations” of the AFM…  You are right that total payload is people, baggage, fuel, etc. However there are further limitations on how that can be carried, where on the aircraft that further limit the total number.

TLDR: aviation mission planning is complex.
It's not rocket science. Oh wait... it is!
It's the Rocket Equation, actually.
The keystone fact you're missing is that cars weigh 3000 lbs and have 100 lb. of usable fuel, which is trivial, so you're used to dismissing fuel as a factor.  But on an airplane, fuel is a huge part of total aircraft weight, and has a huge effect on mission capability.  Further, total weight has a huge impact on fuel consumption, so you get into a vicious cycle where the weight of the fuel impacts the amount of fuel needed, carrying more fuel burns more fuel because of induced drag, and here we are at the ever-annoying exponentiality of the Rocket Equation.
As such, calculations are done to determine how much fuel to carry.  You need extra for surprises, but other than that, the less the better for efficiency.  Unless the price of fuel at airports differs enough to justify intentionally "tankering", and now we add the dismal science to the fray.
Suffice it to say there is no lack of complexity.
Cargo capacity is impacted by fuel
The Wikipedia page stated a cargo capacity, and you took it on faith that it meant "with full fuel tanks".
Well, that would be a fairly ... crippling... way to rate an aircraft.  The reality of the Rocket Equation is that you can trade range for cargo capacity, and why shouldn't you if the airplane can handle it?
This results in a cargo rating which is, yeah, not at maximum range rating and vice versa.
Anyway, since you're not actually on a rocket,  your answer is "plan fuel stop(s)".  Or if impossible, get a bigger airplane.

Maximum takeoff weight can be restricted by a variety of aircraft performance constraints -- short runway, sloped runway effects, warm or hot outside air temperature, nearby obstacles that need to be cleared and the combination of all of those. The lighter the aircraft, the better the performance.
Furthermore, for certification (=safety) reasons, most business jet takeoff performance calculations assume an engine loss just prior to liftoff and subsequent single engine climb performance to clear obstacles. All constraints must be met under that assumption. What airport are you planning to takeoff from?

I don't think the PC-24 performance calculations assume losing an engine after takeoff.

It'll still be climbing past 20,000 feet on one engine.


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What does "E" stand for in a polar curve?
In the following polar curve, there is a symbol "E" with the number 33.5 next to it. What does this mean?

E is used as the symbol for L/D in German. Normally, a symbol description will call it "Gleitzahl" (glide ratio).


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Distance of downwind leg from runway in glider landing pattern?
Is there guidance from the FAA or AGA on the distance of the runway from the downwind leg of a landing pattern for a glider?
For powered aircraft, the FAA recommends in its flying handbook (FAA-H-8083-3B) that the downwind leg be flown from 2500 to 5000 feet out from the runway, a pretty wide variance.

I was taught (by various publications and compenent CFIs) that angles are a better thing to focus on than distances.  For example, this publication suggests:

You  should  turn  from  your  crosswind  leg  (or  from  your  45  entry)  onto  your downwind leg when the runway centerline, or an extension of it, is 25-30° below the horizon. Using angles automatically compensates for variations in altitude.

For me, this makes sense for several reasons:

Judging angles is much easier than judging distances.
The AGL pattern altitude might differ depending on the airport/field you are landing at.
If you are landing out, you should ALWAYS fly a normal pattern if possible.  This may force you to fly an abbreviated pattern closer to the ground.

Obvously, adjustments to this will need to be made for a crosswind.

What is meant by the term "landing out?"

@Terry In gliders, "landing out" refers to landing somewhere that isn't an airport, due to not having enough altitude to reach an airport. Wikipedia says it's a routine occurrence in cross-country soaring.

Not true. We humans are very, very bad at estimating vertical angles. Ask any random person about the angle of a road or mountain slope, and you'll typically get a guess exaggerated up to an order of magnitude. That said, as pilots, we can train ourselves and learn cues to get the angle right, such as the classic 'fly such that the runway crossed the wing strut about here' (on a Cessna).

According to the FAA's Glider Flying Handbook (chapter 7):

The distance for a normal pattern from downwind leg to the landing
  area should be approximately one quarter to one half of a mile. Of
  course, this depends on current conditions and the type of glider.
  This varies at different locations.

You may wish to start with the safe envelope parameters of your glider for turning in a landing pattern, in other words, speed and bank angle.  A glider turning a half circle at 50 knots (V 1.2 stall) will be much closer to the runway on downwind than a glider turning at
150 knots (V 1.2 stall). From the diameter of your safe 1/2 circle glide (and altitude loss),
you determine your safe minimum distance from runway on downwind.  Of course, you can come in a little higher (and a little farther away) and "square it off" into a downwind, 1/4 turn, base leg, 1/4 turn, and final.
But the safe turn is the key.  Too slow or too tight could lead to a bad situation near the ground.  Not a good time to stall.  Hopefully your airport is "glider friendly" and a suitable landing pattern can be worked out.


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What happens if the pilot tries to do a loop in a full-motion simulator?
I know that full-motion simulators, typically used to train pilots on commercial aircraft, have limits to how much they can tilt. They can't go upside down, for example. What happens if the pilot goes outside those limits, by doing a loop for instance? Does the simulation continue with the loop and the just the physical tilt effect gets ignored, or does the whole simulation shutdown and reset?

during a full loop the perceived gravity is still  in "pilot's down" direction: the pilot is still siting on the seat rather hanging by the harness. In this case, gravity is actually doing you a favor to simulate centrifugal force. The tough case is when the plane is flying upside down and that should be the scenario in question.

@user3528438: To achieve a nice, round loop it is helpful to push at the highest point, so the pitch rate is reduced and negative gs are quite likely. When I did my first flight in a Boeing Stearman, I did just that, which resulted in urgent "pull, pull" shouts from the instructor in the back seat. It turned out he hadn't secured his pockets and was starting to loose their content in the open cockpit. Given the low g limits on airliners, looping them will certainly involve negative gs at the top.

The motion systems in simulators do NOT mimic the airplane's attitude. Whether the airplane is right-way-up, upside-down or inside-out has no effect on the motion system.
The motion system is used to move local gravity around to mimic the accelerations felt while flying. eg. To mimic the braking during landing, the cab tilts forward, moving local gravity forward relative to the cab, making it feel like you're hanging in the straps due to the braking. Note that when you are braking during landing, the cockpit is level, and yet you are still propelled forward.

You're right in that gravity is used for simulating sustained accelerations by tilting the platform. Onset linear accelerations are reproduced like in the real vehicle, it's just that that has the whole world to move around in and the motion system has only a metre or so. You're also absolutely correct in that the motion system reproduces accelerations. We're geared to feel those, and get sick when we expect them and cannot detect them.

No they cannot do loops. During a loop, the pilot feels an increase in effective gravity (n). A motion system can only increase Z-forces by going up, but has to stop this when the travel stops are reached. It then goes imperceptibly back to neutral position, while the manoeuvre continues without the motion system being able to contribute to it.
So the motion system will tilt and heave during the initiation manoeuvre for the loop, will then heave some more until actuator stroke runs out, and them slowly return to neutral.
Motion systems are great for commercial airliners, which cannot do loops. For fully aerobatic aircraft such as military jets, G-suits and G-seats are used that simulate secondary g-force effects:

A G-seat lowers the seat pan slightly and loosens shoulder straps when pulling g.
A G-suit pumps up air chambers around the trouser legs, so that the pilot feels the blood flow restriction that reduces blood being drained from the brain, thereby delaying unconsciousness.

G-suits are used in the actual jets, pilots wear them inside the simulator as well.

I am interested in what happens in actual flight simulators. (Also, BTW some commercial airliners are capable of doing loops. The ULF for Boeing 737, for example, is about +3.75g which is the same needed to do a loop.)

Actual flight simulators for military jets often have no motion systems. In a simulator with a motion system, it just returns to neutral during a loop.

At Flight Safety in a Class D simulator, the aircraft I have flown there will do aerobatic maneuvers, such as spins, loops and barrel rolls, but they will not provide realistic motion feedback.  However the simulator visual display and instruments will emulate the aircraft, as the dynamic requirements exceed the design limits of the motion simulation.

I think the curious point of the question is, what exact will it do? We know it will not provide realistic feedback, but will it like, bounce around or shake or ?

It provides motion feedback until the excursion limits are exceeded then it will resume the axis exceeded after about 100 msec.  One time I had a the DG tumble so many aspects of the sim are realistic.  The programming changes periodically on the sims.  On one recurrent ride the sim was smoother entering a flat spin than on a previous ride. I had the same instructor and he commented that they made SW changes as a military customer who bought training in that aircraft had made some requests.


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How does jet training work for private pilots? Are there jet trainers?
For a private pilot (not an ATP) that wants to get ratings for jets, how does that work?
What kind of training aircraft is used typically?

Related, related

Possible duplicate of Is it legal to begin pilot training on a jet?

There are a number of simulator training companies that will provide instruction and simulators in an attempt to provide a type rating to a private pilot.  The training is not cheap, even for ATP candidates who are proficient, so expect additional costs as more sim time will most likely be required to be proficient to pass the ATP/Type Rating test.

Cirrus will do it but I think you have to buy one of their jets. You could train in a BonusJet if you want to fly the SubSonex.

Possible duplicate of Is there any flight school that uses jets as part of their training program?

There is no FAA requirement for any specific pilot certificate level beyond Private to get a type rating in any jet. In fact, many owner/operators of small business jets (CJ, 500s, Phenom, etc) only have a Private certificates.
See 14 CFR 61.31
The only FAA requirements are that you have a Private certificate with multiengine and instrument ratings. Additionally, a High Altitude Endorsement is required to take the actual type checkride unless you meet the grandfather requirements of the HAE. RVSM is NOT required to take the checkride, but it is required before flight into or through RVSM airspace.
You can enroll in a simulator based Part 142 course (think Flight Safety, Simcom, LOFT) and complete your entire training in a Level C or D full flight simulator, but once you complete your training you will have an SOE limitation on your type rating. This is per 14 CFR 61.64. This regulation says you must have 25 hours of Supervised Operational Experience in the aircraft before you can act as PIC in that aircraft unless you meet specific requirements including total time, time in multi-engine turbine (including turboprop) aircraft, military training, etc.
You can enroll in a part 61 course and complete your training in the actual aircraft and once you pass the type rating, you would not have the SOE limitation under 14 CFR 61.64 since you took the actual checkride in the aircraft as opposed to an FFS.
You can enroll in a Part 142 initial hybrid program that offers simulator training for the first part and when you have complete the sim portionof your training they move you to the aircraft for the final training segments and checkride. This also eliminates the 61.64 limitiation on your certificate.
Three things to remember: First, ALL type ratings are done to ATP standards regardless of certificate level and Second, SOE or not, insurance companies rule on how much time you will need to fly with a mentor pilot before going out on your own and finally an in-aircraft checkride for your type rating will be far easier than an in in-sim checkride.


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Why would a commercial flight be landing with a tailwind?
I just flew into BOS on flight 2539 and we landed either on 22R or 22L even though winds were 19 knots at 010 at the ground and 14 knots at 300 at 3000 feet. There was no change either; the captain had announced northerly winds 30 minutes prior as we approached the destination.
Why would they be landing with a tailwind? Normally flights land at 4L or 4R when winds are like this.

There are a number of reasons to land with a tail wind at a large commercial airport. Two of the most common are operational consideration and noise abatement. At LAX for example the standard pattern is to land and depart to the West (24L/R and 25 L/R).  In order to "turn the boat" there is a 15 minute period where no one is departing or landing.  With planes coming in at one per minute per complex you can end up with 30 planes waiting to land and needing to get to the other side of the airport and another 30 on the ground.  Not something to do for a 3kt tail wind.
The approach for LAX is over the city.  From 0000 to 0715 the airport operates with both arrivals and departures over the water, if conditions allow (and in the early morning that is often the case.)  I live under the SMF departure for 16L/R and the airport will operate against the wind up to 5kts at night to keep planes away from our neighborhood, but unlike LAX we have a prevailing wind that blows up the river from San Francisco at night so we rarely get to benefit from these noise abatement procedures.


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If an aircraft decompresses at altitude will everyone start farting?
From what I understand about how gases and liquids work, if the environment loses pressure, then it causes dissolved gases to come out of solution. For example, if a diver ascends too quickly, dissolved nitrogen will come out of solution causing the "bends".
I have heard that astronauts undergoing decompression training experience gastrointestinal distress from the effect.
Does this mean that if an aircraft at altitude suddenly decompresses that everybody will start farting?

I'm voting to close this question as off-topic because this question is better on health.SE.

That could speed up evacuation when on the ground, and contribute to safety.

"Does this mean that if an aircraft at altitude suddenly decompresses that everybody will start farting" Some may sneak out while I'm filling my pants at whatever emergency is going on, but I haven't heard of that. Some people I know say flying makes them gassy, but sounds more like an excuse to me.

@RonBeyer Gas in your intestines will certainly expand, causing discomfort or pain. That even happens to some people as a result of normal cabin altitudes. So I wouldn’t say it’s just an excuse. Whether or not that gas can find its way out is another story.

If gas is all that comes out it wouldn't be too bad.

For those over fifty, it may take on forms other than gaseous ...

@kevin Being on-topic on some other SE site isn't a reason to close here. If you want to claim it's off-topic here, you need to give a reason why it's off-topic here, not why it's on-topic somewhere else.

@TomMcW Well, more precisely, the pressure gradient will increase. Since your intestines aren't completely rigid, they might expand a bit, but not necessarily by much.

If this were the case, wouldn't it already happen when the cabin altitude climbs during the initial climb on every flight in a pressurized aircraft? Also, seems like if this were a serious concern, we'd have heard about it from the many examples of aircraft depressurizing at altitude.

@reirab I’ve read that some people have complaints about gas on flights. Can’t say exactly where I read it, but I came across it while reading about the differences between the 6000ft cabin alt in A380 and the 8000ft alt in other aircraft. I don’t think it’s a “serious” concern, not like the bowel is going to burst or anything. It can just cause discomfort in some people. In a sudden decompression event a minor stomach cramp would prob be the least of a person’s concerns.

Not an answer because it's hearsay but I had a checkride with a 737 pilot who told me he'd had a rapid decompression at 35k feet.  He said he farted, belched, his ears hurt and the windows and instruments were instantly covered in condensation.  I donn't know how typical this is

No, they will not.
In the case of a rapid decompression, an aircraft will make a very rapid descent to a altitude, and while I'm not a physician or an expert on these physiological and biochemical processes, I doubt very much that would be enough time for dissolved gases to be released into the digestive tracts.
Even if it were, a person who experiences a sudden accumulation of gas in the gastrointestinal tract may experience discomfort or even pain, but it takes a while for gas to make its way to an exit - as anyone who has suffered from trapped wind will know.

After a night in Wiesbaden, beer and bratwurst was consumed in great quantities.  The morning ride in the high altitude pressure chamber was quite odiferous upon explosive decompression.  So, it is possible.

Ditto with Mike Brass' comment.  I have made a few trips to 25K' in the chamber and felt the need to toot.  It isn't just dissolved gasses or the time it may require for them to come out of solution, your gastro-intenstinal tract already has actual gas in it that will expand anytime the outside pressure decreases.  Not everyone will suddenly fart uncontrollably, but many will feel some level of discomfort from the expanding gas and choose to let go.


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Is there any comprehensive source for landing fee information?
Is there any comprehensive source for landing fee information for US airports?
Airnav and the facility directory do not have landing fee information.
Also, going to airport web sites often is useless. For example, Massport lists landing fees for Hanscom on their web site, but do not list their fees for Logan.
Note that landing fees can be complicated. For example, there can be "landing fees" and "ramp fees" and others. So, usually the fee information is for a bunch of different stuff.

I'm not sure that there is, the problem is that landing fees/ramp fees are business information and set by the airport/FBO management. I doubt that the FAA (who publishes the Chart Supplement, the new name for AFD) would want to get involved in, just like fuel prices. Larger carriers may have negotiated fee structures with the airport as well. It would be nice if somebody consolidated the basic fees like they do for fuel though.

PFC (Passenger Facility Charge) fees are published by the FAA, but these are separate from basic landing fees and tiedown fees. From the FAA Airport Compliance Manual Order 5190.6B, there is some guidance about how to determine landing fees. Not necessarily something the FAA is maintaining comprehensive data on.

The Airport/Facility Directory only states if there is a fee, not the amount. At military airports, the fees are fixed by law, but civilian airports have no such restriction. In other countries such as Germany the law states exactly how much is charged.

See "Resource Location" under what not to ask in [help].

After working on the problem some more I have found there is not really any centralized source for landing fee information. Pretty much the only option is to call the FBO. However, you can get the FBO information from Flight Aware, assuming a US airport. For example, to get FBO information for Logan Airport in Boston, you would use the URL:
https://flightaware.com/resources/airport/KBOS
This tells you that the GA FBO for Logan is Signature and that their number is 617-561-2500. At that point you have to call them to find out more. For example, I did this and the answer was:
\$33 single engine piston landing fee
\$6 per hour parking
\$53.90 general landing fee
\$49.00 handling charge (waived if you buy at least 15 gallons of fuel) \$95 "security" charge
\$5 random bureaucracy charge

Is the fee actually called a "random bureaucracy charge" or is that something you made up?


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Is there a modern single-seat turboprop?
There seem to be a number of tandem seat turboprop trainers like the PC-9/Texan and the Super Tucano. Are there any single-seat turboprops? I am asking about modern production planes.

Yes. Although most aircraft to find good use for a turboprop need to have more than one seat, some single-seat uses do exist. For example, agricultural aircraft like the Air Tractor AT-802, Thrush models, and the PZL-106 Kruk are all currently in production.


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Why do fighter pilots wear helmets?
I notice that fighter pilots wear helmets, although pilots of aircraft like C-130s do not. What is the purpose of this?
I have heard that it could be helpful in the event of an ejection, but considering the explosive force of an ejection, if you hit your head I would be surprised if a helmet made any difference.

What if:
(a) shrapnel hit the aircraft;
(b) the pilot hit his/her head BEFORE he/she ejects;
(c) the pilot hit his/her head AFTER he/she ejects. There are a lot of events an helmet comes in handy whe flying a tactical aircraft...

Some ejection systems only fracture the window, not jettison the canopy.

When ejecting at high speed, having a helmet with full face mask is handy protection once the pilot is in the free air stream - who know what parts of plane debris or incoming ammo (s)he might encounter. It's a handy way of distributing the weight of the headset & O2 mask around the entire head. Its a good place to display HUD information (newest of new systems, only). They look cool.

For combat missions and for high-altitude drops when the crew needs to be on oxygen, C-130 pilots wear helmets too. But for routine, pressurized (or low altitude) flights, headsets are more comfortable. But for anybody sitting on an ejection seat, helmets it is -- take any advantage you can during one of those!

@Ralph Helmets have nothing to do with oxygen supply.

We had quick-don masks for O2 when wearing the headsets, but those weren't allowed for high altitude airdrops -- helmets were required. In that airplane, quick-don masks were considered for emergency use only, and any planned use of O2 involved the helmet + mask.  More modern quick-don masks are better than the ones we had, so as a change in policy you might could wear them for peacetime high altitude work, but that's what the policy was "back when."

I realize that oxygen systems are integrated into the helmets, but it is completely feasible to make an oxygen system that has no helmet, unless full body pressurization is needed, which as far as I aware is only relevant for specialized aircraft like the SR-71 and U-2. My question concerns ordinary fighter aircraft like F-16 and F-18 that operate below 60,000 feet.

It's possible after an ejection that the pilot isn't conscious. Might be good to protect the noggin after he falls over when hitting the ground.

There are several reasons:

A helmet provides head protection. A fighter airplane can make many sudden turns and a helmet provides reduction of the risk of a head injury.
If the pilot needs to eject the airplane, helmet is needed to protect from wind blast, in addition to what I mentioned #1.
Helmets play an important role in noise attenuation.
The mic and headphones are mounted in the helmet.
Many helmets contain a sun visor, hence the pilot does not require to use sunglasses.
Modern helmets have a mounted display, night vision support. The latest ones help to see the pilot in any direction they desire, and even through the airplane.
Oxygen supply mask can be directly mounted to the helmet.

I think it would be worth mentioning the issue of shrapnel protection mentioned in other answers.

Or the more mundane loose object during negative G maneuver

In addition to ejection it's to protect the pilot's head from impacts against the inside of the cockpit in case of sudden maneuvering or loss of control. Many aerobatic pilots wear helmets for this reason as well.

Primarily to protect the pilot's head from windblast and and trauma from blunt object impact in the event of an ejection and parachute landing.  The current generation helmets do not offer ballistic protection against enemy fire

There was at least one case when a helmet saved a helicopter pilot's life, namely a RAF's Chinook HC4 pilot. In short, Flight Lieutenant Ian Fortune was injured while extracting some soldiers in Afganistan. An enemy bullet basically struck his helmet above the upper rim, crushing it and sending parts of the helmet and visor into his face, but nevertheless saving his life and leaving him more or less operational and conscious.
Fun fact: the helicopter he was flying, serial number ZA718, has enough stories to tell to have his own nickname and Wikipedia page :).

Keep in mind that a pilot can eject at 500kts+, or possibly even supersonic.
The helmet and faceplate keeps the pilot's face from being seriously damaged by high speed airflow. Helmet also protects the pilot's skull if they strike the seat after separation, or when they impact the ground... same purpose as a motorcycle helmet... the human skull can be fractured by as little as a 15mph impact, and head injuries tend to be more serious (or fatal) than similar impacts elsewhere on the body.
Today, the helmet also doubles as a HUD mount, with the faceplate being the screen. In the F35, the helmet/HUD combination is reported to cost over $400k, but it does give the pilot to 'see' through the aircraft... tracking opponents that would otherwise be blocked from the pilot's view.

If somebody ejected at supersonic speed the helmet would just increase the chance of having their head ripped off.


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Can the CG move out of bounds as fuel is used?
I've never bothered to check the weight and balance when the tanks are empty. I always assume that if the Center of Gravity is within range when the tanks are full, then it will remain within range as the flight progresses.
Am I correct? Should I compute the W&B on both ends?
I know some airliners have complex fuel systems which require fuel to be moved around during flight (e.g. B747-100, Concorde). I am interesting in small aircraft in general, i.e. General Aviation and perhaps personal and up to regional jets.

I was recently flying a DA20, fully fueled up for 4 hours of flight time, where the endurance ended up being 3:30h, because of exactly this problem with the CG being too far FWD if the fuel tank was totally empty.

Should I compute the W&B on both ends?

Yes, you should generally always compute W&B on both ends of the flight. If for no other reason that you may want to know where your CG falls when you land as even within limits it can greatly change handling of the aircraft. If you are currently training you should encourage your instructor to setup situations where you can fly the aircraft at different loads, perhaps bring a friend along to ride in the back and fill the tanks or you and your instructor a very light fuel load and some pattern laps.

I always assume that if the Center of Gravity is within range when the
  tanks are full, then it will remain within range as the flight
  progresses.

This depends on the airframe and you should consult the POH that accompanies the airframe. It really depends on the tank configuration, the fuel load, and the aircraft load. For example the Bonanza has tip tanks that actually increase useful load which can have a pretty big CG impact. While some GA planes have their tanks mounted fairly center mass and the burn has little impact.
In the various PA-28's and 172's I have flown, generally with myself and a passenger, they are fairly hard to push out of CG limits just by burning fuel. However there are plenty of bigger GA planes that have more complex fuel systems, and extra bag compartments that allow potential CG issues.

C172 is certified for both the normal and utility categories, but with more stringent weight and CG limitations for the latter. It is easy to stray from one category to another as the fuel burns, although in practice you'll usually move into the utility area. Also, from memory, on C152 it is possible to have a too-forward CG with two heavier pilots and nearly empty tanks.

With a straight wing aircraft with all the fuel in the wings, the fuel is close enough to the C of G across the span of the tanks that the C of G shift as fuel is consumed is insignificant.  When this is the case, the weight and balance chart won't have any special requirements to show C of G shift, relative to the fore/aft limits, from fuel burn.
If there is an aux tank that is forward or aft, the effects of its fuel level on C of G will be included in the airplane's weight and balance, and there may be limits on loading of cabin or cargo incorporated into the weight and balance data, and special instructions for C of G calculations when it's being used along with curve added to the wight and balance graph.
On swept wing airplanes, sweep and dihedral in wing tanks causes C of G to move forward as fuel is burned (level drops in the tips first, which are aft).  On swept wing airplanes with a center tank, the center tank is almost always burned first and it causes an aft shift of C of G as its fuel is burned, followed by a forward shift as the wings are consumed.
The C of G charts of swept wing airplanes will include a fuel burn curve to show the C of G location variation with fuel burn. If no center tank, the curve will slant or curve to represent the forward shift, and if there is a center tank it will have a kind of sideways V shape as the C of G moves aft and then forward as the pounds of fuel load declines.
Bottom line is, do whatever the weight and balance chart for the airplane says to do.  If it's something like a 172, there isn't going to be any major shift. If it's something like a Taylorcraft with a nose tank, C of G will shift aft as fuel is burned.  To keep things simple, there may be a baggage restriction that is intended to prevent exceeding the aft limit when the tank is empty at the end of a trip.

It's not general aviation, but the P-51 had enough of a CG shift as fuel as used to be worthy of mention in the Pilot's Handbook (reference)

The fuselage tank should be used for takeoff and climb to to a safe
  altitude as it is the most direct system to the engine and is on a
  higher plane in relation to the engine.  Use of the fuselage tank will
  also move the C.G. of the airplane forward to a more desirable
  position for flight.

and also

The stability of the plane improves rapidly as fuel is expended from
  the fuselage tank.


text:
Why did Concorde not have horizontal stabilizers?
The Concorde did not have horizontal stabilizers or elevators. As a result, the pilots have to move fuel around the aircraft to trim it up or down, as sluggish and hazardous move.
Why did the design team chose not to use a horizontal stabilizer? Was it because of the understanding of supersonic aerodynamics and/or technology of the time? Or, was it due to some aerodynamic principles? If someone were to design the Concorde today, would they still use the fuel transfer method to balance the CG?

Perhaps related

On how fly-by-wire reduces requirement of fuel shift and allowing more fuel to be loaded before taking off: https://www.flightglobal.com/FlightPDFArchive/1978/1978%20-%201060.PDF

Interesting article from Australia  "NASA has inked a deal with Lockheed Martin to develop a supersonic “X-plane” that could break the sound barrier without a sonic boom."  http://www.news.com.au/travel/travel-advice/flights/nasa-announces-319-million-supersonic-plane/news-story/15e03a2b76cd6a44196b9fa48f79803f  And it has small horizontal tail surface.

And these guys seem to be making progress towards a next generation supersonic plane, with a 2 seat flyable prototype being assembled in Colorado https://boomsupersonic.com/xb-1  You would probably have better luck asking their engineers why no horizontal tail surface  https://boomsupersonic.com/xb-1  https://boomsupersonic.com/static/images/xb1-progress-2.jpg

(Source: concordesst.com)
It does have elevators in the form of elevons at the trailing edge.  A delta is effectively a tailless flying wing with a really long chord.  Like any flying wing, pitch stability is achieved by down force generated at the local trailing edge by a control surface that does the same job as a regular elevator/stab, by applying down force to balance the forward pitching moment of the CG ahead of the centre of pressure (an airplane is really just a teeter totter with the C of G at one end and the elevator at the other).
Fuel transfer is used to keep the C of G in a sweet spot range to minimize trim drag, which is the penalty taken in generating the down force at the rear, so that the down force can be no more than necessary for adequate pitch stability.
As for why they didn't use a horizontal stabilizer, the reason is that the delta plan form with elevons provides the required stability and control in pitch and allowed the designers to dispense with a separate tail surface, greatly reducing the drag of a separate tail (not eliminating, because there is still a drag penalty from the elevons generating down force).  Same reason anybody goes with a flying wing.  Plus, you need lots of sweep for low supersonic drag and lots of wing area for high altitude efficiency, so a tailless delta fits the bill.
A limitation of this is a narrower CG range (a more limited forward CG than with a regular tail that is farther aft and has more leverage) because the elevons have a short moment arm, a limitation of all flying wings.  The ability to transfer fuel to adjust CG would help with this and I think Concorde's modest passenger loading didn't cause the CG range issues that a normal airliner would have to deal with.

Because horizonal stabilators are there to balance the pitch moment from the main wing lift, which, in a conventional aircraft, (where the main wing center of pressure is behind the aircraft center of gravity), would cause the aircraft to pitch nose down. The horizontal stabilator at the tail creates a down force, which creates a nose-up pitching moment equal to the nose down pitch moment from the main wing. The down force at the tail is significantly less than the Lift from the main wing (but further away from the CG) so the moment is creates balances the Main Wing Moment. Realize that the above description is a simplification, as actually, all of the forces are actually pushing on each tiny little bit of surface area of the aircraft wings, fuselage, horizontal stablator, everywhere, and the description above is just an arbitrary way of splitting up the effects of all those tiny individual forces into a few major pieces, which are equivalent to what is really happening, so we can understand and talk about it all with some understanding.
On a Delta wing, without a horizontal stabilator, exactly the same thing is happening, but it is now the result of the distribution of all the tiny lift forces over the entire surface of the one single delta wing. Over most of the wing, except for near the trailing edge, the forces are pushing the aircraft up, nd because most of the wing is behind the CG, rotating the nose down. But the trailing edge of the wing is curved upwards slightly, so that far back from the CG, the forces on the wing are pushing the aircraft down with a smaller force, but much longer moment arm (further away from the CG, and this rotates the nose up by the proper amount to balance the nose down pitching moment from the rest of the forces forward on the wing.


text:
Which aerodromes are in the scope of ICAO recommendations?
I'm looking for a clarification of how aerodrome characteristics are regulated in a country which has signed the ICAO Chicago Convention (more on Wikipedia). To guide the answer, I'm providing a list of ICAO statements, which may be ignored if non pertinent. The question follows this list.
ICAO statements

Under this convention, State members participating to international air travel agreed to follow ICAO recommendations and best practices:

Article 12: Each contracting State undertakes to keep its
  own regulations in these
  respects uniform, to the greatest possible extent, with those
  established from time to time under this Convention. Over the high
  seas, the rules in force shall be those established under this
  Convention. Each contracting State undertakes to insure the
  prosecution of all persons violating the regulations applicable.

The States also agree to provide some infrastructure:

Article 18: Each contracting State undertakes, so far as it may find
  practicable,
  to: a) Provide, in its territory, airports, radio services,
  meteorological services and other air navigation facilities to
  facilitate international air navigation, in accordance with the
  standards and practices recommended or established from time to time,
  pursuant to this Convention.

Nineteen annexes describe SARPs, standards and recommended practices. Annex 14 (volume 1) describes aerodromes. Applicability is defined in § 1.2:

1.2.2 The specifications, unless otherwise indicated in a particular context, shall apply to all aerodromes open to public use in
  accordance with the requirements of Article 15 of the Convention.

Article 15:

Every airport in a contracting State which is open to public use by
  its national aircraft shall likewise, subject to the provisions of
  Article 68, be open under uniform conditions to the aircraft of all
  the other contracting States.

Article 68:

Each contracting State may, subject to the provisions of this
  Convention, designate the route to be followed within its territory by
  any international air service and the airports which any such service
  may use.

Each State member also publishes aeronautical information (AIP), described in Annex 15 to the convention:

2.1.1 Each Contracting State shall: a) provide an aeronautical information service (AIS); or b) agree with one or more other
  Contracting State(s) for the provision of a joint service; or c)
  delegate the authority for the provision of the service to a
  non-governmental agency, provided the Standards and Recommended
  Practices of this Annex are adequately met.

Each State must list in the AIP the deviation from ICAO SARPs, in section GEN:

4.1.1.1 c: a list of significant differences between the national regulations and practices of the State and the related ICAO Standards,
  Recommended Practices and Procedures, given in a form that would
  enable a user to differentiate readily between the requirements of the
  State and the related ICAO provisions;

From the statements listed, it's not clear if all aerodromes open to the public are in the scope of ICAO recommendations and SARPs. This may be only those open to international travel, there is some ambiguity.
Question
How do I know whether such aerodrome in a given country benefit from ICAO recommendations as included in the national regulations. For example, there is this recommendation (annex 14):

3.9.6 Recommendation.— To facilitate the movement of aeroplanes,
  fillets should be provided at junctions and intersections of taxiways
  with runways, aprons and other taxiways. The design of the fillets
  should ensure that the minimum wheel clearances specified in 3.9.3 are
  maintained when aeroplanes are manoeuvring through the junctions or
  intersections.

How does that apply to aerodromes in Germany or in Canada for instance?

This is a great question, but is also very broad. Except in answering specifically for the examples you ask for—Germany and Canada—I think the best answer would be: The scope of the applicability of the ICAO recommendations depends on what the signatory country determines it to be.

@JonathanWalters: This is why I suggest these cases, to write a practical answer that is pedagogical enough to extrapolate to other countries.


text:
Are new airliners less safe than older the two first years after they are marketed?
Looking at the 2014 Allianz Global Aviation Safety Study for airliners:

The last generation of aircraft (1974 and forward, including A320 and B737) have a very low rate of accidents: less than 2 accidents per million of departures. This rate is better than all previous generations.
However this rate is lower only one to 2 years after the model has been on the market. Prior to this milestone, the rate is higher than previous generations, except the very first one.

Why is the first year rate of accidents so relatively high? It seems we have almost the same number of accidents before two years, than during the rest of the aircraft life.
Is there an obvious reason for having aircraft of the last generation apparently less safe at their life beginning than the two previous generations?

For all generations, this looks rather like a bathtub curve. But the numbers here are small, so I'm not sure how to draw good conclusions from them.

Just a thought - when a new aircraft is introduced, if there is an accident/incident early on there are very few aircraft of that model flying and the million hour mark is hard to reach.
This may skew the accident rate wildly.  Earlier generations were simpler and were turned out of the factory at a higher rate than newer, more complex aircraft.

The amplification effect plays a role in both cases: If accidents happen late in the period required to have 1M flights, the ratio is much lower during this period. So your answer is valid if the accidents happen early. So my first question: Are aircraft less safe at the beginning? On the other hand can 12 to 24 months be the time for having enough aircraft for 1M flights per year (2,700 flights per day) to explain much lower and stable rates?

As time goes by, more experience and knowledge accumulates. This should increase the safety with each generation.  The increasing complexity on the other hand, leaves more chance for so-called "infant failures" to creep into the operation.  As I said, just a thought.


text:
Which was the first pressurized aircraft?
Which aircraft was the first one to fly pressurized?

High altitude breathing apparatus, 1918 (source)
I mean an aircraft which didn't require to don an oxygen mask above a certain altitude, excluding very high altitude where a mask is still required due to quick loss of consciousness after a depressurization event.


text:
What prevents a Machmeter from being the standard airspeed indicator?
Airspeed is more accurate when displayed by a Machmeter than when displayed by an airspeed indicator. It isn't affected by ambient temperature / density changes. It is still affected by position error, hence it must indicate something between IAS and CAS.

Source, source
What does prevent to have a Machmeter scale both in knots and Mach numbers, and use the Machmeter in place of the airspeed indicator, as the standard airspeed instrument?
Such instruments exist:

Source (From FAA Airplane Flying Handbook)

I assume you're asking about larger turbine aircraft? Being 10 knots too fast is significant when landing a light aircraft, but it's only about M0.01 and I'd hate to have to try to tell the difference between M0.1 and M0.11 on that Machmeter in your question :-)

@Pondlife: No, I'm asking in general. The problem you point out isn't a problem of unit or instrument, just a problem of difference between low/high values which is more large in high-speed aircraft. You would have the same problem with an ASI without a moving scale.

Yes, it was mostly a tongue-in-cheek comment. But practically speaking, dealing with hundredths of a unit in flight plans, documentation etc. seems like it would be rather awkward, even if the instrument shows it clearly on an appropriate scale.

"It isn't affected by ambient temperature / density changes." - I question this assertion, given that the speed of sound itself varies with both temperature and altitude.  So you tend to gain nothing accuracy-wise, as the mach number still won't accurately tell you how fast you're traveling unless you manually correct for ambient factors.

@aroth, I took into account a Mach meter shows ${EAS} / {k\sqrt {P_s}}$, and EAS isn't dependent on local density or local temperature. Is this wrong? If not, wouldn't that mean, given M is proportional to EAS, a Mach meter is more useful for the pilot than an ASI, at subsonic speed (e.g. at 300 kt)? I agree it's not more accurate, but more useful. I used more accurate because some measuring biases had been removed.

I don't know, my thinking is more intuitive/theoretical than functional.  If I start from the (unrealistic) assumption that a machmeter accurately reports the exact mach number, then it seems that it's giving an inaccurate (or at least, "tricky") approximation of airspeed because 'Mach 0.8' means something different at ground level than it does at 35,000 feet.  However, if the functional implementation of the machmeter relies upon fundamentally the same inputs (subject to the same errors) as an ASI, that tends to imply that the accuracy would be comparable, yes.

Machmeter-based tachometer would not indicate something between IAS and CAS, it would indicate TAS but for the same¹ instrument and position errors the normal airspeed indicator has.
However, airspeed indicator does not indicate true airspeed and this is intentional.
The EAS, that the airspeed indicator is aiming to show, is a measure of dynamic pressure. Since lift, which also includes effectiveness of control surfaces, is proportional to dynamic pressure and drag is also a function of dynamic pressure, most handling characteristics depend on the dynamic pressure. Therefore the value indicated by the airspeed indicator is exactly what the pilot wants to know.
The only effects that depend on other speeds are aeroelastic flutter, which depends on true airspeed, and compressibility effects, that depend on Mach number. Both limit the maximum safe speed. That is why fast aircraft also need Machmeter. But they still need the indicated² airspeed.

¹ Both instruments use the same two raw values obtained from the pitot-static system: total and static pressure. The airspeed indicator shows square root of the difference (because dynamic pressure is proportional to square of speed), the Machmeter shows the ratio to some fractional exponent. So all the errors affect both the same way, and in electronic instruments, they are compensated² to the same extent.
² Electronic systems in modern airliners say to show CAS, correcting all errors they can reasonably estimate.

Could the indicated mach number be calibrated at sea level pressure, similar to IAS?

"Therefore the value indicated by the airspeed indicator is exactly what the pilot wants to know." Wants to know or needs to know?

@FreeMan, I would stick with wants, because there are workarounds for when he does not, so it's not an absolute necessity, just very, very useful.

Just making sure!

...Machmeter scale both in knots and Mach numbers

The simple answer is that unlike say (MPH vs KTS) the equivalents change with altitude so one scale would have to move. For example at sea level Mach 1 is roughly 661 Kts. by comparison at 35,000 ft its 574 Kts. Even though an air speed indicator also suffers from similar corrections the MPH/KTS scales are always in line. The IAS/TAS calibration creates a similar issue but generally its acceptable to correct once cruise altitude has been reached (or you are at least in a cruise climb situation and free to handle such correction). The truth is that most modern glass cockpit units handle this for you.
One other thing to consider is units. Mach Number, strictly speaking is unit-less while speed is measured as Distance/Time many might say these two things cant be on the same instrument.
The fact remains that you, could, potentially print them on the same instrument as long as one scale was constantly adjusted for. I presume (based on the picture) that Mach gauges may have an issue with low readings which could be a problem for things like stall labeling etc.
You may also want to take a look at the example outlined here.

"at sea level Mach 1 is roughly 661 Kts. by comparison at 35,000 ft its 574 Kts", this is a comparison of Mach number and TAS, IAS and TAS conversion has the same problem, still we use IAS.

I have added some more to possibly clarify

The dual ASI/Machmeter shown in the OP appears to have a movable Mach scale, so that may be exactly what's done on these dual gauges.

I noticed that as well I need to do some research on how they work and what they are as I have never seen one before.

The needle would move very little at lower airspeeds and would be subject to non-linearity. Velocity follows directly from $p_{total} - p_{static} $, while Mach number is a fraction. From Wikipedia:
$$M = \sqrt{5\left[\left(\frac{p_t}{p_s}\right)^\frac{2}{7}-1\right]}$$
At the beginning of the scale, there is very little relative change and very little movement of the needle, and as the speed picks up there is more needle movement per knot.

A plot of $\Delta$ dynamic pressure as a fraction of static pressure (at sea level in ISO atmosphere) shows this. At 200 knots, the dynamic pressure is only 6% of the static pressure. Any friction in the measuring equipment will have large effects on the accuracy and linearity of the speed reading up to this point. Only at speeds of > 200 knots does the gradient start to be reasonably useable.

@JanHudec I could not find a fault in this answer

Hm, WP says $M = \sqrt{5\left[\left(\frac{p_t}{p_s}\right)^\frac{2}{7}-1\right]}$. And so does the other answer.

Yes, perhaps a better source, hey? I'll update.

@JanHudec I don't know where they got that from. I've made an Excel sheet for different speeds at sea level, dynamic pressure = $\frac{1}{2} \cdot \gamma \cdot p_s \cdot \left( \frac{V}{a}\right)^2$. Up until M=0.5 the WP equation is spot on, and then starts to deviate

Unfortunately I don't know where they got it from and while I know quite a bit of physics, fluid thermodynamics involving $\gamma$ seriously confuses me.

@JanHudec I posted a question on physics SE, the answer turns out to be pretty simple. Have also placed a comment at the wrong answer referenced above.

@JanHudec a concise and elaborate derivation here


text:
Can an engine that is extinguished by foam be returned to service?
During the Qantas 32 accident, one of the engines could not be shut down as the control cable from the cockpit to the engine is cut. The fire crew eventually shut down the engine by shooting fire-fighting foam into the engine.
In such cases, can the engine be returned to service following an inspection? Or, does foam damage the engine to the extent that it is more economical to simply buy a new engine?


text:
Is it possible to obtain a CPL without a PPL?
I've overheard that trainee pilots hired by airlines are not required to pass the PPL exam before attempting the CPL exam. Is this allowed by regulation?

Some countries have multi crew competency(?) certificates for pilots who only have airline training.  They can fly an Airbus, as SIC, but can't fly a small airplane.

Because the answers can be significantly different between different sets of regulations (and there are major differences in pilot licensing rules in this case), this should really be three different questions.  Already, we have two different answers from two different people covering the different aspects, and both cannot be accepted as "correct".  I suggest that you narrow the scope of this one and ask two more questions if you want a definitive answer for each.

You've tagged your question with 3 different sets of regulations, I can't answer all of them but here are the FAA ones.
The answer depends on what you mean by "exam". If you mean the written test, then anyone can take it if they have a graduation certificate or a logbook endorsement:

An airman applicant may present one or more of the following
  acceptable forms of authorization for all tests listed above [...]

Certificate of graduation or a statement of
  accomplishment certifying the satisfactory completion of the ground
  school portion of a course for the certificate or rating sought. The
  certificate or statement may be issued by an FAA certificated pilot
  school or an agency, such as a high school, college, adult education
  program, Civil Air Patrol, or Reserve Officers Training Corps (ROTC)
  flight training school.
Written statement or logbook endorsement
  from an authorized ground or flight instructor certifying that the
  applicant completed an applicable ground training or home study course
  and is prepared for the knowledge test. (AC 61-65E) (14 CFR § 61.35)

At least in theory, you could get a certificate or endorsement to take the commercial written test without ever having taken any other FAA test. But in practice I suspect that it's very unlikely: why would an organization or CFI endorse you as ready for the commercial written if you haven't already done the private one?
However, if "exam" means the actual checkride then the situation is much clearer: it isn't possible to do it unless you already have a private certificate, at least for civilian pilots. 14 CFR 61.123 says:

§61.123   Eligibility requirements: General.
To be eligible for a commercial pilot certificate, a person must:
  [...]
  (h) Hold at least a private pilot certificate issued under this
  part or meet the requirements of §61.73

61.73 lets you use military experience to apply for a civilian certificate.
Finally, some people use the term "commercial pilot" when "ATP" would be more accurate. I'm not saying that you're using it incorrectly, but since I have no idea how EASA and CASA define a CPL it may be worth mentioning. In the US, ATP multi-engine candidates need a certificate for the written exam, and a commercial certificate with instrument rating (or military equivalent) for the checkride. See 61.153.


text:
Which air traffic controller positions are considered most challenging?
Consider a large ATC facility, where a range of ATC services are provided and each position requires a dedicated person to manage (unlike small airports, where the same guy is ground + tower). Which positions are considered the most demanding, and it would make sense to assign a senior and experienced controller there?
I would imagine, Clearance Delivery is probably the easiest one, then Enroute, Approach / Departure, Ground and finally Tower. What's the training sequence recommended by the authorities for a new controller? After all, assigning a fresh trainee to LAX tower or JFK ground does not sound like a good idea.

I'm guessing you can find lazy approach control sectors somewhere in the world, just as you can burn yourself out doing clearance delivery at Heathrow in a matter of minutes.

As worded, this question is essentially asking for opinions. I think you could ask the question without appealing to opinion, though I'm not sure it could be answered objectively. I'm not ready to vote to close, but do think the question needs a rework.

Possible dupe and onother possible dupe


text:
Which regulator should I follow to comply with ADs?
If some component was manufactured by an American company and installed on Airbus aircraft operated by a Canadian-registered airline. Should the operator check American ADs, or they will be adomted by EASA or Canadian civil aviation authority?

Typically you need to follow the directives and regulations of whatever authority the aircraft is operating under.
In Canada that'd be Canadian authorities. If the aircraft were to be used internationally, it'd also be any country's authorities that the aircraft crossed and lands in.
Of course in most cases when it comes to ADs, national authorities will all enforce them, whichever one issues it (not counting authorities in backwater countries like Zimbadwe or Cambodia probably, if they even have authorities of their own and don't just simply follow the regulations of a larger neighbour country).

if AC and component are made by BAC, then AD must be followed FAA rules, if the AC fly to Europe, then EADAS rules must be followed also.

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